The present invention relates to a sealing assembly for a moving mechanical member, in particular a rotary shaft of a hydraulic pump, and which is suitable for use at both high and low external temperatures, and especially in the presence of wide angles of displacement of the shaft and in the presence of friction bearings.
Various types of seals for sealing the power takeoff side of hydraulic pumps are known, and which are inserted between the rotary shaft and the bearing to separate the incoming fluid from the outside environment, e.g. the compartment of a vehicle engine or any other sort of power generating unit.
In particular, double-lip sealing assemblies are known in which a first or so-called "dust cover" lip--mainly for protection against external agents--is fitted on the air (outer) side, and a second lip--which performs the actual sealing function--is fitted on the fluid (inner) side.
Currently marketed multiple-lip seals, however, involve several drawbacks.
Firstly, such seals fail to allow, with no loss in sealing performance, for the adaptation of wide angles of displacement of the shaft rotation axis, as in increasingly common applications in which conventional rolling bearings are replaced by friction bushes to permit considerable radial slack.
Secondly, such seals adapt poorly to changing operating conditions: as is known, sealing performance is affected both by the temperature and pressure of the fluid in the pump circuit, and by the environment, particularly the outside temperature.
The conditions in which seals are called upon to operate may therefore differ widely. For example, overpressure of the operating fluid may occur, thus resulting in compression of the sealing lips towards the air side, and in a so-called "extrusion" effect capable of detaching the dust cover lip.
On the other hand, negative pressure of the incoming fluid, caused, for example, by low outside temperature, may raise the viscosity of the fluid, which therefore requires a considerable amount of energy for its movement, generates intake resistance, and, along considerable portions of the pump-tank conduits, produces low-pressure states in the pump in proportion to the fall in temperature of the fluid.
Consequently, when turned on, the pump tends to reduce the pressure on the fluid side of the shaft seal, and attempts to draw in air through the main sealing lip, which therefore tends to lift.
One known solution to at least partly eliminate the above drawback is to design the dust cover lip to also act as an air valve, which closes when the low-pressure state extends from the fluid side to a cavity specially formed between the two lips.
This solution, however, is only effective at low temperature when the seal is made of low-glass-transition-temperature rubber (e.g. silicone or nitrile rubber), which, on the other hand, does not normally conform with requirements in terms of wear resistance, fluid compatibility and resistance to high temperatures.
In other words, a compromise is inevitably sought between the chemical and physical characteristics of the elastomeric material used, and the construction design of the seal, both of which, however, invariably limit the performance of the seal to some extent.
It is an object of the present invention to provide a sealing assembly for rotary shafts, designed to overcome the aforementioned drawbacks typically associated with known seals. More specifically, it is an object of the present invention to provide a sealing assembly capable of maintaining a high degree of efficiency even in the presence of wide angles of displacement of the shaft, and which is equally effective at high and low operating temperatures using low-glass transition-temperature elastomeric materials.